1. Field of the Invention
This invention relates in general to electronic systems and in particular to the reduction of EMI emissions in electronic systems.
2. Description of the Related Art
Computer systems are information handling electronic systems that can be designed to give independent computing power to one user or a plurality of users. Computer systems may be found in many forms including, for example, mainframes, minicomputers, workstations, servers, personal computers, internet terminals, notebooks, and embedded systems. Computer systems include desk top, floor standing, rack mounted, or portable versions. A typical computer system includes at least one system processor, associated memory and control logic, and peripheral devices that provide input and output for the system. Such peripheral devices may include display monitors, keyboards, mouse-type input devices, floppy and hard disk drives, CD-ROM drives, printers, network capability card circuits, terminal devices, modems, televisions, sound devices, voice recognition devices, electronic pen devices, and mass storage devices such as tape drives, CD-R drives, or DVDs.
Electronic systems inherently generate electromagnetic (EM) fields. All EM fields are originally sourced by active electronic components in the system, but are delivered into space as Electromagnetic Interference (EMI) by various system structures which act as antennas. One source of EMI is from differential and common mode EM fields from traces and planes of circuit boards.
The Federal Communications Commission (FCC) in the United States and other international agencies set standards for the maximum intensity of electromagnetic energy which can be radiated and conducted in specific electromagnetic bands from a computer system. Shielding can be used to reduce EMI emissions from a computer system. However the greater the intensity of EM fields generated by the system, the higher the required shielding efficiency (SE) of the shield if needed to reduce EM emissions. Increasing the SE typically is accomplished by increasing the thickness of the metal panels of a metal shield or increasing the thickness of metallic coating on the outer plastics of the computer system, either of which increases the cost, complexity, and/or weight of the system. Furthermore, designing a shield with a high SE requires that interface structures be built within high tolerances to prevent EM waves from leaking through the shield via openings in the shield, such as openings for receiving external data and power signals. These high tolerance interface structures also add to the cost and/or complexity of a computer system.
Because of these and other disadvantages, it is desirable to reduce the EM radiation generated by the signal traces of an electronic system. Examples of techniques for reducing EM radiation include capacitive filtering of clock leads, proper termination and routing techniques of signal traces, and preventing, where possible, the signal traces from crossing splits in the reference planes. Examples of techniques for reducing EM radiation are found in U.S. Pat. No. 6,150,895 entitled xe2x80x9cCircuit Board Voltage Plane Impedance Matching,xe2x80x9d having listed inventors Todd Steigerwald and Mitchell Sebring, and a common assignee, and in U.S. Patent Application entitled xe2x80x9cImproving the EMC Characteristics of a Printed Circuit Board,xe2x80x9d having listed inventor Jeffrey C. Hailey, a common assignee, and a serial number of 09/400,025, now U.S. Pat. No. 6,349,038, both of which are hereby incorporated by reference in their entirety.
What is needed are additional techniques for reducing EMI radiation.
It has been discovered that providing a phase shift between signals having a common frequency component transmitted on adjacent portions of circuit board signal traces advantageously reduces EMI emissions from those signals.
In one aspect of the invention, a circuit board includes a first signal trace for coupling to a first circuit to provide a first signal to the first circuit. The circuit board includes a second signal trace for coupling to a second circuit to provide a second signal to the second circuit. The first signal and the second signal have a common frequency component. The first signal trace and the second signal trace each include adjacent portions being located adjacent to each other on a signal layer of the circuit board. The first signal trace includes a first delay element located generally towards a first side of its adjacent portion opposite a first circuit side of its adjacent portion to provide a phase shift between the first signal transmitted on the adjacent portion of the first signal trace and the second signal transmitted on the adjacent portion of the second signal trace to reduce electromagnetic interference emissions due to signals propagating on the adjacent portions of the signal traces.
In another aspect of the invention, a computer system includes a processor, a memory operably coupled to the processor, and a circuit board. The circuit board includes a first signal trace coupled to a first circuit to provide a first signal to the first circuit and a second signal trace coupled to a second circuit to provide the second signal to the second circuit. The first signal and the second signal have a common frequency component. The first signal trace and the second signal trace each include adjacent portions being located adjacent to each other on a signal layer of the circuit board. The first signal trace includes a first delay element located generally towards a first side of its adjacent portion opposite a first circuit side of its adjacent portion to provide a phase shift between the first signal transmitted on the adjacent portion of the first signal trace and the second signal transmitted on the adjacent portion of the second signal trace to reduce electromagnetic interference emissions due to signals propagating on the adjacent portions of the signal traces.
In another aspect of the invention, a circuit board includes a first signal trace for coupling to a first circuit to provide a first signal to the first circuit and a second signal trace for coupling to a second circuit to provide a second signal to the second circuit. The first signal and the second signal have a common frequency component. The first signal trace and the second signal trace each include adjacent portions being located adjacent to each other on a signal layer of the circuit board. The circuit board also includes means for providing a phase shift between the first signal transmitted on the adjacent portion of the first signal trace and the second signal transmitted on the adjacent portion of the second signal trace to reduce electromagnetic interference emissions due to the signals propagating on the adjacent portions of the signal traces.
In another aspect of the invention, a method for making a circuit board includes providing a first signal trace with an adjacent portion and a first delay located generally towards a first side of the adjacent portion. The first signal trace is positioned to carry a first signal to a first circuit. The method also includes providing a second signal trace with an adjacent portion and a second delay located generally towards a second side of the adjacent portion. The adjacent portion of the second signal trace is adjacent to the adjacent portion of the first signal trace. The second signal trace is positioned to carry a second signal to a second circuit. The first signal and the second signal have a common frequency component. The first and second delays are sized to provide a phase shift between the first signal transmitted on the adjacent portion of the first signal trace and the second signal transmitted on the adjacent portion of the second signal trace to reduce electromagnetic interference emissions due to the signals propagating on the adjacent portions of the signal traces and to provide the first signal to the first circuit generally in sync with the second signal being provided to the second circuit.